Recently, in the field of wireless communication, multiple input multiple output (MIMO) systems which transmit multiple independent transmission signals from a radio transmission device to a radio reception device at the same frequency and timing so as to implement high speed transmission have been attracting attention. MIMO may increase the transmission rate without increasing the frequency bandwidth.
In the MIMO systems, different transmission signals are multiplexed and received by a radio reception device. Thus, technology for separating spatially multiplexed transmission signals is necessary for the radio reception device.
FIG. 12 is a schematic diagram showing a conventional MIMO system. The MIMO system includes a radio transmission device 51 and a radio reception device 52.
The radio transmission device 51 includes T (T is an integer greater than or equal to 2) transmission antennas As1 to AsT. The radio transmission device 51 transmits different transmission signals s1 to sT from the transmission antennas As1 to AsT to the radio reception device 52.
The transmission signals s1 to sT transmitted from the transmission antennas As1 to AsT of the radio transmission device 51 are received by R (R is an integer greater than or equal to 1) reception antennas Ar1 to ArR provided in the radio reception device 52. At this time, transmission and reception signals of the MIMO system may be expressed as shown in Equations (1) to (5).
                    r        =                  Hs          +          n                                    (        1        )                                r        =                              [                                          r                1                            ⁢                                                          ⁢              ⋯              ⁢                                                          ⁢                              r                R                                      ]                    T                                    (        2        )                                H        =                  (                                                                      h                  11                                                            ⋯                                                              h                                      1                    ⁢                                                                                  ⁢                    T                                                                                                      ⋮                                            ⋱                                            ⋮                                                                                      h                                      R                    ⁢                                                                                  ⁢                    1                                                                              ⋯                                                              h                  RT                                                              )                                    (        3        )                                s        =                              [                                          s                1                            ⁢                                                          ⁢              ⋯              ⁢                                                          ⁢                              s                T                                      ]                    T                                    (        4        )                                n        =                              [                                          n                1                            ⁢                                                          ⁢              ⋯              ⁢                                                          ⁢                              n                R                                      ]                    T                                    (        5        )            
In this regard, a vector r of the left side of Equations (1) and (2) is an (R rows×1 column) reception signal vector having, in elements, reception signals received by the respective reception antennas Ar1 to ArR of the radio reception device 52.
A vector H of the left side of Equation (3) is an (R rows×T columns) propagation channel matrix having, in elements, propagation channel responses h11, . . . , hR1, . . . , h1T, . . . , and hRT among the transmission antennas As1 to AsT of the radio transmission device 51 and the reception antennas Ar1 to ArR of the radio reception device 52.
Here, h11 is a propagation channel response between the transmission antenna As1 and the reception antenna Ar1. hR1 is a propagation channel response between the transmission antenna As1 and the reception antenna ArR.
Also, h1T is a propagation channel response between the transmission antenna AsT and the reception antenna Ar1. hRT is a propagation channel response between the transmission antenna AsT and the reception antenna ArR.
The right superscript T of a matrix indicates a transpose matrix of the matrix.
A vector s of the left side of Equation (4) is a (T rows×1 column) transmission signal vector having, in elements, transmission signals transmitted by the respective transmission antennas As1 to AsT of the radio transmission device 51.
A vector n of the left side of Equation (5) is an (R rows×1 column) noise vector having, in elements, noise added to the respective reception antennas Ar1 to ArR of the radio reception device 52.
A linear process is known as a technique of separating signals spatially multiplexed as shown in Equation (1). For example, the linear process is zero forcing detection (ZFD) or minimum mean square error detection (MMSED).
The linear processes are widely used since a calculation amount is small. The above-described MIMO system is disclosed in Non-Patent Document 1.
To obtain good transmission characteristics using the above-described linear process like ZFD or MMSED, it is preferable that the relationship of M≦N should be established between the number of transmission antennas, M, of the radio transmission device 51 and the number of reception antennas, N, of the radio reception device 52. If M>N, the transmission characteristics are significantly degraded.
To avoid this problem, it is desirable to increase the number of reception antennas of the radio reception device 52. However, if the radio reception device 52 is a small-size radio reception device, there is a problem in that it is difficult to increase the number of reception antennas since the number of reception antennas capable of being mounted is limited.
Non-Patent Document 1: D. Gesbert, M. Shafi, D. Shiu, P. J. Smith, A. Naguib, “From Theory to Practice: an overview of MIMO space-time coded wireless systems”, IEEE JSAC, April 2003